Molecular imaging of small laboratory animals, such as mice and rats, is a vital tool in the study of disease. Mouse models of human diseases, such as cancer, Parkinson's disease, and Alzheimer's disease, are providing important clues to the causes, diagnosis and treatment of these, and many other, disorders. Ultra-high resolution PET and SPECT have enabled the visualization of biochemical processes in vivo in small animals. Despite the important results obtained using pinhole SPECT, it is clear that the sensitivity of SPECT systems is sub-optimal. Some sensitivity can be recovered by either increasing the pinhole diameter, resulting in a loss of resolution, or increasing the injected dose, which may lead to radiation damage in the subject. This project will develop novel, widely applicable, non-invasive methods for small animal SPECT, using multiple-pinhole techniques to improve the image statistics, while maintaining the spatial resolution, and potentially reducing the radiation dose to the animal. Various multiple-pinhole configurations will be investigated, including both overlapping (in which the projection images overlap on the detector face), and non-overlapping (in which the geometric arrangement of pinholes is designed to produce separate images on the detector). Monte Carlo computer simulations will be used to optimize the pinhole arrangement, providing high-resolution images, without compromising the signal-to-noise ratio or introducing image artifacts. The optimum design will be application-dependent, and determined using quantitative techniques and Hotelling observer studies. Additional image reconstruction improvements, such as correction for depth-of-interaction in the crystal, and pinhole septal edge penetration, will be investigated using Monte Carlo simulations. Model-based iterative reconstruction techniques will be developed to deconstruct the data from the multiple-pinhole systems, and to incorporate the corrections for pinhole edge penetration and depth of- interaction. The optimized multiple-pinhole system will be constructed and compared against the single pinhole device in both phantom and animal studies, to validate the multiple-pinhole approach, and to test the reconstruction algorithm. The improvements in the imaging technology proposed in this application will lead to enhanced sensitivity, and improvements in the throughput of small animal molecular imaging due to the decreased imaging time required. It also takes advantage of adapting existing clinical SPECT machines, rather than developing a dedicated small animal imaging system, which makes it much less expensive and more widely available. [unreadable] [unreadable]